Because an MgB2 superconductor, which was discovered in Japan in 2001, has the highest superconducting critical temperature (39 K) among the known metal-based superconductors, and because it is relatively easy to form the MgB2 superconductor in a bulk or wire material, many studies have been made all over the world on the development of bulk or wire MgB2 superconductors. A method of manufacturing the bulk materials generally includes compacting a mixed powder of Mg or MgH2 powder and B powder under pressure and heat-treating (sintering) the compacted product. One major method for forming the superconducting wire is a powder-in-tube method in which a mixture of Mg or MgH2 powder and B powder is filled in a metal tube, followed by rolling and a heat treatment. The critical current density (Jc) of the bulk or wire material produced by the known powder-sintering method or powder-in-tube method is about 3,000 A/cm2 at 4.2K and 10 T, and about 700 A/cm2 at 4.2K and 10 T and, thus, is not very high.
With a view toward improving the critical current density (Jc) of MgB2 superconductors an attempt has been made to incorporate an additive into a mixture of Mg or MgH2 powder and B powder. For example, addition of SiC powder having a particle size at the nanometer level (10 nm to 100 nm) is reported to be effective to obtain an improvement of the critical current density (Jc) (S. X. Dou, et al., Journal of Applied Physics 94 (2003), 1850 (Non-Patent Document 1); H. Yamada et al., Journal of Applied Physics 19 (2006), 175 (Non-Patent Document 2); and H. Yamada et al., Journal of Applied Physics 20 (2007), 1 (Non-Patent Document 3)). Further, Non-Patent Document 2 discloses that an aromatic hydrocarbon, particularly benzene, is effective to improve the critical current density (Jc) of an MgB2 superconductor.
However, the critical current density (Jc) of the MgB2 superconductor obtained from a mixture of Mg powder or MgH2 powder, B powder and SiC is at most about 22,000 A/cm2 at 4.2K and 10 T, and about 10,000 A/cm2 at 10 T. On the other hand, the critical current density (Jc) of the MgB2 superconductor obtained from a mixture of Mg powder or MgH2 powder, B powder and benzene is at most about 13,000 A/cm2 at 4.2K and 10 T, and about 4,000 A/cm2 at 10 T. In this circumstance, there is a strong demand for an MgB2 superconductor having a higher critical current density (Jc).